drm/i915: Convert mmio_flip::seqno to struct request

Converted the mmio_flip 'seqno' value to be a request structure as part of the
on going seqno to request changes. This includes reference counting the request
being saved away to ensure it can not be retired and freed while the flip code
is still waiting on it.

v2: Used the IRQ friendly request dereference call in the notify handler as that
code is called asynchronously without holding any useful mutex locks.

For: VIZ-4377
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Thomas Daniel <Thomas.Daniel@intel.com>
[danvet: Drop the _irq variant and use the normal reques unref,
wrapped in dev->struct_mutex per the discussion on the m-l.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>